PSI - Issue 17

Joyraj Chakraborty et al. / Procedia Structural Integrity 17 (2019) 387–394 Joyraj Chakraborty/ Structural Integrity Procedia 00 (2019) 000 – 000

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2.2. Measuring system

The sensors were installed in the cross-section at the southeast end of the bridge during the erection of the works in 2015. The location of sensors is marked in red and green in Fig. 3. The middle part of the girder was chosen to be monitored, as the biggest stress changes encounter in there. It can be divided into four groups of measurement systems to determine the changes in the superstructure. In the first group, new ultrasonic sensors were mounted in the middle of the span over a length of 4 meters. Each of the eight ultrasonic sensors can work as a transmitter and a receiver.

Fig. 3. Arrangement of ultrasonic sensors and strain gauges in the girder.

Fig. 4 shows the exemplary signal (0,0) from pair 1 – 2 and its spectrum, which is representative of all the signals in duration and bandwidth. The center frequency of the measured signal is around 60 kHz with a sampling frequency of 1 MHz corresponding to the multiple scattering regime. The recorded duration of 5 ms corresponds to about 20 m of travel for this mode, allowing many reflections to occur and resulting in a like diffuse wave. Therefore, the recorded duration is sufficient to acquire most of the reverberating energy. It allows to locate and determine the changes in the structure based on the velocity of the ultrasound wave.

Fig. 4. Diffuse ultrasonic signal and its spectrum.

Three Geokon ® 4202 sensors were installed in each of the bearing pads. Their purpose is to measure the forces transmitted by the bearing to the concrete of the sub-bearing blow, so they are used to determine the correct operation of the bearing. Six Tascas ® thermistors and three Geokon ® 4911 sensors were installed in cross-section A-A (see Fig. 3). The second group, located in the B-B cross-section (see Fig. 3), consists of three Geokon ® 4202 sensors.